Magnetic transducing head



March 15, 1966 D. s. OLIVER MAGNETIC TRANSDUCING HEAD 4 Sheets-Sheet 1Original Filed Jan. 9, 1961 DONALD $.OLIVER VENTOR. BY AT TORNEY muvzwmwum h QED-2042 March 15, 1966 D. s. OLIVER MAGNETIC TRANSDUCING HEAD 4Sheets-Sheet 2 Original Filed Jan. 9, 1961 FIG.3

DONALD S- OLIVER INVENTOR. 2 ATTORNEY.

March 15, 1966 D. s. OLIVER MAGNETIC TRANSDUCING HEAD 4 Sheets-Sheet 5Original Filed Jan. 9, 1961 DONALD S. OLIVER INVENTOR.

BY AT ORNEY.

March 15, 1966 D. s. OLIVER MAGNETIC TRANSDUCING HEAD 4 Sheets-Sheet 4Original Filed Jan. 9, 1961 DONALD s. OLIVER INVENTOR. BY ATTORNEYUnited States Patent 3,240,881 MAGNETIC TRANSDUCING HEAD Donald S.Oliver, West Acton, Mass., assignor to Itek Corporation, Lexington,Mass, a corporation of Delaware Continuation of application Ser. No.81,326, Jan. 9, 1961.

This application Mar. 11, 1965, Ser. No. 438,883 2 Claims. (Cl.179-1002) This is a continuation of application Serial No. 81,326, filedJanuary 9, 1961, now abandoned. This invention relates generally to dataprocessing systems and more particularly to reading and erasing magneticrecord data. The term reading as used herein included read-in, e.g.,magnetic recording, and read-out, e.g., playback of magneticallyrecorded data.

Prior art systems for magnetic recording and playback utilize anelectromagnetic transducer or head rereciprocally to convert electricand magnetic signals. The head typically comprises an electromagneticcoupling means, such as an induction coil, coupled to a loopshapedferromagnetic core terminating in a pair of gap defining pole pieces. Amagnetizable storage medium such as commonly known magnetic tape isplaced in magnetic coupling proximity with the gap. Coupling between thehead and tape takes place by intersecting the fringe magnetic field inthe vicinity of the gap. The fringe field extends outside the gapdefining pole pieces and intersects the plane defined by the oppositeedges of the pole pieces at the outer periphery of the core.

The maximum information track width of the magnetic tape is defined bythe width of the gap in proximity with the tape. To increase the gapWidth the head height is increased, as, e.g., by adding additional corelaminations. For a fixed track width the height of prior art headscannot be reduced below the width of the track. For many applications itis desirable to have a more compact and thinner head.

One common type of head consists of a plurality of flat rings formedfrom a low remanence ferromagnetic material. The rings are laminatedtogether with their fiat surfaces in face to face relationship toprovide a permeable magneticcore. The core has a radial gap formedtherein, and a coil is toroidally wound around a portion of it. Anelectric current passing through the coil produces magnetic flux in thecore and consequently a magnetic flux across the length of the gap. Amagnetic storage medium such as'magnetic tape is brought into couplingproximity with the fringing flux in the vicinity of the gap in a planeperpendicular to the plane of the rings. For recording the fringing fluxin this plane magnetizes the tape in proportion to the current in thecoil. The width of of the magnetized track produced is determined by thewidth, i.e., the height of the head. The peripheral dimensions of thecore in the plane of the ring are principally determined by the spaceneeded to accommodate the toroidal winding previously mentioned.

For playback, magnetized signals of varying amplitude in the tape arecoupled to the core through the pole piece edges and induce electricalsignals in the winding. The transducer thus reciprocally convertsmagnetic and electrical signals.

In many applications it is desirable to reduce the size of the recordinghead while not sacrificing the recording track Width. For example, inthe copending application of Manfred R. Kuehnle, Serial No. 30,928,filed May 23, 1960, now abandoned, there is described and claimed amagnetic recorder using a magnetic tape which is in the form of an edgecoiled helix. The Kuehnle application is assigned to the assignee ofthis application. The electromagnetic transducer is placed betweenadjacent helical 3,240,881 Patented Mar. 15, 1966 turns of the helix andbrought into contact with the magnetic surface of the tape. In order toobtain maximum data density in a given volume of tape, it is desirablethat the adjacent helical turns of the tape be closely spaced.

One limitation on the spacing of the turns is the volume of the headthat must be inserted between the turns for recording, playback, orerasing. It is, therefore, desirable that the head occupy as small avolume as practicable. In addition, it is desirable that the head itselfshould be thin enough so that it may be inserted between the adjacentturns of the tape for random access to the data stored on individualloops of tape. Prior art heads, because of their size, do not lendthemselves to this type of application. Such a head has a height atleast, equal to the width of the recording track and a depth sufiicientto accommodate the necessary toroidal windings. Therefore, with theprior art head one cannot obtain the thickness of head desirable forrandom access, nor the compactness of head for data storage density.

In known heads the functions of recording, reproduction and erasing havebeen accomplished by using heads of widely divergent designs. However,all known heads operate on the principle that the plane of the loopshaped ferromagnetic core, or in the case of certain erase heads theplane of the permanent magnet, is substantially perpendicular to theplane of the storage medium at the gap. Thus the useful fringe fluxintersects a plane perpendicular to the core or magnet.

In the present invention the electromagnetic transducer includes a loopshaped ferromagnetic core. The core defines a surface patch and an axisnormal to the surface patch. The term surface patch as used hereinincludes a generalized surface or part of a surface bounded by a closedcurve in contradistinction to a surface of infinite extent. The term isused to cover various core configurations that are included within thescope of the invention. The loop shaped core provides a low reluctanceclosed loop flux path. Around one portion of the lamination a coil ofwire is toroidally wound. The core terminates in a pair of gap-defining,spaced, juxtaposed pole pieces. The pole pieces are preferably formedout of the plane of the surface patch. It may be desirable to adjust thelength of the gap by inserting therein a shim of non-magnetic materialhaving a thickness equal to the desired gap length. The whole assemblymay then be stiffened and potted using known techniques. In operation anelectromagnetic transducer as just described is placed adjacent to astorage medium having a flat magnetic data storage surface whereby theline of contact between the magnetic storage surface and the gap isparallel to a plane tangent to the surface patch in the vicinity of thepole pieces. The fringe flux intersecting this parallel plane isoperative on the storage media. Therefore, the combination of the headand the data storage medium is very compact.

It is therefore an object of the invention to provide an improvedelectromagnetic transducer exhibiting greater compactness for a givenwidth of recording track.

It is another object of the invention to provide an improvedelectromagnetic transducer that is thinner than known electromagnetictransducers for a given informa tion track width.

Still another object of the invention is to provide an improved magneticdata processing system exhibiting greater compactness and simplicity ofstructure.

Yet another object of the invention is to provide a magnetic readinghead providing a range of information track article comprises a loopshaped ferromagnetic core means means defining a surface patch and axisnormal to the patch. The core means defines a low reluctance closed loopflux path substantially coextensive with its loop shape. High reluctancemeans are coupled to the core means in series with the flux path. Thepole pieces are so shaped as to provide at least one pair of opposededges in a plane transverse to the axis and parallel to a planetangential to the surface patch in the vicinity of the pole pieces. Thepole pieces provide fringe fiux lines intersecting the plane. Inaddition, electromagnetic coupling means are provided. Theelectromagentic coupling means are coupled to the core means, and incooperation with the fringe fiux reciprocally convert electrical andmagnetic signals.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription taken in connection with the accompanying drawings and itsscope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is partially schematic diagram, of a data processing system,including an article embodying the present invention.

FIG. 2 is a sectional view taken along line 2-2 in FIG. 1;

FIG. 3 is a perspective view of an electromagnetic transducer embodyingthe present invention.

FIG. 3A is a sectional view taken along lines 3A-3A in FIG. 3;

FIG. 4 is a plan view illustrating a modfication of the electromagnetictransducer in FIG. 3.

FIG. 5 is a sectional view of the embodiment illustrated in FIG. 4 takenalong line 55 in FIG. 4;

FIG. 6 is a perspective view illustrating another modification of theelectromagnetic transducer in FIG. 3;

FIG. 7 is a perspective view illustrating still another modification ofthe electromagnetic transducer in FIG. 3;

FIG. 8 is a perspective view illustrating yet another modification ofthe electromagnetic transducer in FIG. 3;

FIG. 9 is a front elevational view, partially schematic, of a dataprocessing system embodying the present invention;

FIG. 10 is a sectional view taken along line 1010 in FIG. 9; and

FIG. 10A is a sectional view illustrating a modification of the systemillustrated in FIGS. 9 and 10.

Referring now to the drawings, and with particular reference to FIG. 1there is here shown a partially schematic diagram illustrating a dataprocessing system embodying the present invention. In FIG. 1 there isshown an erase head 20, record head 21, and playback head 22, made inaccordance with the present invention. The record head and playback headare shown as two separate heads in this drawing for purposes ofillustration only, it being understood by those skilled in the art thata single head may be used for either recording or playback.

The data processing system shown in FIG. 1 broadly illustrates amagnetic tape recorder system. For recording, an input signal is coupledthrough a microphone 24 and amplifier 25 to a record head 21. The head21 is disposed in magnetic coupling proximity with a magnetizable datastorage medium or tape 32, as shown. The head 21 includes anelectromagnetic coupling means 26b and looped shaped ferromagnetic core27 with a high reluctance gap 31 formed by pole pieces 29 and 30. Anerase head 20 including an electromagnetic coupling means 260 andferromagnetic core 38 is coupled to the tape 32.

The outer perimeter of the loop shaped core defines a surface patch, theloop defining a low reluctance closed loop flux path. A bias fiux may beproduced in the core 27 by coupling the core to a D.C. power supply orother means well known in the recording art. The electromagneticcoupling means 26b produces a change in flux in the core 27 inaccordance with the input signal 23. Juxtaposed pole pieces 29 and 30define a high reluctance gap 31 in series with the flux path within core27. The

high reluctance gap causes a fringe flux to appear at the gap. Highreluctance material such as beryllium copper is preferably inserted inthe core gap to insure a reliable gap length.

Referring now to FIG. 2, there is shown a cross-section of core 27 takenthrough the pole pieces 29 and 30 along line 22 in FIG. 1. As can beseen in FIG. 2, the pole pieces are so shaped that a pair of edges 29aand 30a lie in a plane transverse to an axis YY which is normal to thesurface patch defined by the core 27. In addition, the plane of edges29a and 30a is parallel to a plane tangential to the surface patch inthe vicinity of the pole pieces, the trace of the tangential plane onthe plane of the paper being indicated by the line XX. The pole piecesthereby are terminal points for fringe flux lines that interesect theplane of the edges and are outside of the plane. In recording, thefringe fiux varies with the input signal to the induction coil, andmagnetizes magnetic storage medium in accordance therewith.

Referring once again to FIG. 1 and FIG. 2 a magnetic tape 32 istransported across the gap 31 by a tape transporting means 33. The tapetransporting means 33 is adapted to pass the tape across the edges 29aand 30a of the pole pieces 29 and 30 in a plane parallel to XX, at thegap 31, in a direction shown by the arrow A. As shown in FIG. 1, themaximum width of the recording track is the width of the gap 31 asindicated by the dimension B.

The playback head 22 operates in the same manner as the recording headexcept that a portion of the remanattt flux on the magnetic tape isdiverted through the core. The motion of the tape in the vicinity of thepalyback head core gap produces a varying fiux in the core 34. Thechanging flux is converted via the coil 26a into an electric signalapplied through a playback amplifier 35 to a speaker 37, thus the headsmay be used in accordance with the invention to read magnetic data.

To erase, an erase signal 28 is coupled to the core 38 of the erase head20 through an induction coil 260. The erase signal is selected, as wellknown in the art, to erase previously recorded magnetic data. Forexample the erase signal may be a D.C. signal sufiicient to magneticallysaturate the tape.

Referring now to FIG. 3 there is shown an electromag netic transducermade in accordance with the preferred embodiment of the presentinvention. As heretofore stated, the electromagnetic transducer will bereferred to as a head as it is commonly known in the art. The head maybe used either as a recording head, playback head, or erase head. Inparticular, this head will be described in terms of a record-playbackhead. The head broadly includes ferromagnetic core means in the form ofa loopshaped core 40 and electromagnetic coupling means in the form ofinduction coil 41. As illustrated in FIGS. 1, and 2 the core 40 is loopshaped, defining a surface patch. Preferably the coil is formed as asubstantially planar annulus as shown in FIG. 3. The core may be made ofany of the known ferromagnetic materials. By way of example, it has beenfound that a core made of the material known in the trade as HyMu hasbeen satisfactory. The ferromagnetic core 40 provides a low reluctanceclosed loop flux path coextensive with the annulus. High reflectancemeans in series with the core is provided in the form of juxtaposed polepieces 42 and 43, defining a gap 44. The pole pieces are bent out of theplane of the core 40 so that the edges 46 and 47 lie in a plane that isparallel to the plane of the core and transverse to an axis normal tothe plane of the core.

A non-magnetic shim 53, for example .OOOZ-inch thick is placed in thegap 44 for controlling the gap length. The shim is preferably made ofberyllium copper and held in place by a potting compound not shown.Thecore is supported on a support ring 48 which is prefer-- ably madefrom a nonconducting, nonmagnetic material as, for example, plastic. Thesupport ring serves to pre-- vent the core from becoming stressed undulyduring the winding of the induction coil 41.

As shown in FIG. 3a a sectional view taken along the line 3A--3A in FIG.3, the support ring has an annular recess 49. The support ring serves toprevent the core from becoming stressed unduly during the winding of theinduction coil 41. In the preferred embodiment, a coil of wire 50 isplaced around the corners of the support ring so that it contacts thecore along the surface, thus minimizing stresses in the core due towinding. Such stresses may undesirably affect the magneticcharacteristics of the core. The coil 50 is in turn covered by aninsulating tape 51. The coil 50 may be connected to an externalcircuitry, not shown, by a connector 54. By way of example, it has beenfound that a core having a permeability of 25,000 to 30,000 and a coilof 800 turns of #40 copper wire, having an inductance of approximately50 millihenries, are satisfactory for ordinary voice recording orplayback.

The coil 50 is preferably symmetrically disposed about a central axis ofsymmetry in the plane of the core 40 passing through the gap 44 andconnected in humbucking relation to reduce spurious signals generated byexternal magnetic fields. To further reduce effects of external magneticfields, a magnetically permeable shield structure, not shown, may beused in the well-known manner.

The just described embodiment of the invention operates as follows: Whenrecording on a storage medium, a time varying electrical signal to berecorded is applied to the terminals of the coil 50. This electricalsignal is transformed into time varying magnetic flux in the core 40proportional to the original electrical signal. A portion of themagnetic flux appears across the gap 44 as a fringe flux. If amagnetizable storage medium, as for example a magnetic tape shown inphantom in the figure, is passed at a constant rate past the gap 44, thefringe fluxis coupled into the magnetic tape so as to magnetize it in alongitudinal direction. As a result the remanent field in the magnetictape is proportional to the fringe flux which in turn is proportional tothe input electrical signal.

When it is desired to reproduce or read out a signal stored in thestorage medium, the head is used in the opposite way from the way justdescribed. That is, a magnetic tape having a signal stored therein ismoved past the gap 44 at a constant speed, causing a changingmagneto-motive force across the gap 44 which in turn produces a varyingflux in the core 40. This changing magnetic flux induces a correspondingelectrical voltage in coil 50.

Referring now to FIGS. 4 and 5 there is illustrated another embodimentof an electromagnetic transducer or head, made in accordance with thepresent invention. In FIGS. 4 and 5 there is shown a front elevation andtop view, respectively, of an electromagnetic transducer which has thegeneral configuration of an annulus arcuately bent about an axisparallel to a diameter of the annulus. At least two edges of the polepieces are disposed so that the tape intersects the gap along a tangentto the head at the gap as will be more fully described hereinafter.

The electromagnetic transducer includes core means in the form of a loopshaped ferromagnetic core 55 and electromagnetic coupling means in theform of coil 56. The core 55 is formed as an annulus that is arcuatelybent along a diameter, thereby defining a surface patch and a radialaxis, normal to the surface patch. The core 55 provides a low reluctanceclosed loop flux path coextensive with the annulus. High reluctancemeans are provided in the form of juxtaposed pole pieces 57 and 58defining a gap 59. The gap 59 is preferably perpendicular to the axisZ-Z and is along the diameter about which the core is arcuately bent.The edges 60 and 61 of the pole pieces 57 and 58 lie in a planetangential to the core 55 at the gap 59 and transverse to the normalaxis Z--Z.

6. A magnetic tape 62 shown dotted intersects the pole pieces edges 60and 61 in the tangential plane. Although the tape is shown as travelingin a direction parallel to the tangential plane by the arrows C, it willbe apparent that tape need only be traveling parallel to the tangentialplane at the gap 59.

The core is supported on a non-magnetic, non-conducting support ring 63which has an annular recess 64 therein. An induction coil 56, shownschematically, is wound around the support ring and core forelectromagnetically coupling an input signal to :a magnetic tape 62. Thecoil is connected in hum-bucking relation as in FIG. 3, to reducespurious signals generated by external magnetic fields. A magneticallypermeable shield structure, not shown, may be used in the well knownmanner. A nonmagnetic shim, not shown, may be placed in the gap 59 tocontrol the gap length, similar to shim 53 in FIG. 3.

Referring now to FIGS. 6, 7 and 8 there are shown additionalelectromagnetic transducers embodying the present invention. Since theembodiments shown in FIGS. 6, 7 and 8 are generally similar to thoseshown and described in FIGS. 1-5, inclusive, similar parts such assupport rings, coils, insulating tape, shims, etc. have been omitted tomore clearly illustrate the invention. The construction of the omittedelements will be apparent to those skilled in the art from the previousdescriptions and from what is known in the art.

Referring now to FIG. 6, there is shown a loop shaped ferromagnetic coremeans in the form of a substantially square shaped loop ferromagneticcore 70. The core defines a surface patch and a low reluctance closedloop flux path coextensive with the core. In this illustration, the core70 is shown as being formed in two parts 70a and 70b for ease offabrication. This type of construction allows layer type windings to beused. The two parts 70a and 70b are joined together by a lap joint 71.The joint is formed so as to be compatible with the low reluctancecharacteristics of the core. The pole pieces 72 and 73 define a gap 76,and are so shaped as to provide a high reluctance means in series withthe core 70. A pair of edges 74 and 75, which, in operation, contact themagnetic tape lie in a plane substantially parallel to a tangent to thesurface patch in the vicinity of the gap. The electromagnetic transducershown in FIG. 6 has the overall appearance of a thin, fiat, planar loopfor facilitating random access and to provide compactness as previouslydiscussed.

Referring now to FIGS. 7 and 8 there are illustrated alternate forms ofelectromagnetic transducers embody ing the present invention. In FIG. 7the loop shaped core is triangular in shape, while in FIG. 8 the loopshaped core 81 is rectangular in shape. The aforementioned cores areshown for the purpose of illustrating loop shapes embodying theinvention. It will be apparent that many other loop shapes, notspecifically illustrated, also embody the present invention.

Referring now to FIGS. 9 and 10 there is here illustrated, a tapetransport and electromagnetic transducer assembly embodying the presentinvention. FIG. 9 is a front elevational View of the assembly, whileFIG. 10 is a section taken along line 10--10 in FIG. 9. In thisembodiment, there is a thin fiat planar electromagnetic transducer and atape transport means adapted for transporting the tape across a gap inthe transducer so that the tape intersects the gap along a line that istangential to the plane of the transducer. The electromagnetictransducer includes a thin flat planar core 91. The core is in the shapeof an annulus and made of a ferromagnetic material to provide a lowreluctance flux path coextensive with the core. The core 91 is mountedon a non-magnetic nonconducting annular supporting ring 92 having arecess therein for retaining the core. A substantially radial gap 93 isformed in the core by a pair of oppositely spaced juxtaposed pole pieces94 and 95 for providing a high reluctance path in series with the lowreluctance path through the core. The pole pieces are so shaped that atleast one pair of edges 96 and 97 are adapted to intersect the magnetictape 98 tangent to the plane of the core. A capstan 99 mounted in abearing 100 guides the tape and maintains it in contact with the polepieces edges 96 and 97. Although not shown, the capstan may be springbiased to provide a slight pressure on the tape thus insuring positivecontact with the pole pieces edges. The tape is coupled to a transportmeans in the form of a feed reel 101 and takeup reel 102. A feed drive103 is provided for driving a spindle 104 which is coupled to the feedreel 101. A takeup drive 105 drives a spindle 106 coupled to the takeupreel 102. If desired, a single drive may drive both the feed reel andthe takeup reel, as well known in the art. In operation, tape moving inthe direction shown by the arrow D is guided by the capstan 99 tointersect the gap 93 in a plane along a line tangent to the plane of theelectromagnetic transducer 90. Fringe flux existing across the gap, aspreviously described, intersects the tape for providing magneticorientation of the particles thereon to provide indicia of data. Thishead may also be used for playback as previously described withconnection to FIGS. 1-3, inclusive. The electromagnetic coupling means107, shown schematically in FIG. 9, are provided for electromagneticallycoupling an input signal to the magnetic tape through the flux path andto convert magnetic indicia on the tape to electrical signals.

In FIG. A there is shown another embodiment of the invention describedin FIGS. 9 and 10 whereby a single electromagnetic transducer may beused simultaneously to record two magnetic tapes with identical signals,or to playback from two tapes through a single head. Since theembodiment shown in FIG. 10A is similar to that shown in FIGS. 9 and 10,like numerals will be used for like components, and similar componentsindicated by a prime after the numeral. In FIG. 10A two tapes aresymmetrically disposed about the plane of the electromagnetictransducer. Two transport assemblies, not shown, but each being similarto the one shown and described in FIGS. 9 and 10 are provided. Forconvenience of explanation the only portions of the new transportassemblies shown in FIG. 10A are the capstans 99 and 99. The capstansare disposed on opposite sides of the plane of the electromagnetictransducer 90'. The electromagnetic transducer 90 is similar to the oneshown in FIG. 9 except that the support ring is removed along a sectorof the annulus for at least the length of the gap 93. With this type ofconstruction the gap 93 is exposed along both sides of the core 91,whereby two pairs of edges 96-97, and 96'97 are left exposed tointersect with tapes 98 and 98 respectively. Each tape intersects a pairof edges tangential to the plane of the core at the gap. Since thefringe flux intersecting a plane tangential to the core at the gap isidentical along either planar surface of the core 91, the tapes 98 and98' will contain identical lines of magnetic flux thus providingidentical recording on tapes 98 and 98', or simultaneous playback fromtapes 98 and 98. It will also be apparent that this head may also beused for erasing two tapes simultaneously by applying an erasing signalthrough the coil.

It will be apparent that the embodiment shown in FIGS. 9, 10, and 10Aleads to a more compact tape and head assembly than previously known inthe art. Although the head has been described in terms of an annulus itwill also be apparent that other planar shapes such as a C shape, forexample, are also suitable for use in this embodiment. In addition,although the tape has been shown as being spirally wound on spools aswell known in the art, it is contemplated that other tape transportsystems may be used without departing from he concept of the presentinvention, it being only necessary that the tape intersect the gap alonga plane that is tangenial to the plane of the core at the gap. It willalso be apparent that the electromagnetic transducers shown in FIGS. 3through 8 may be used in place of the electromagnetic transducer shownin FIGS. 9 and 10 without departing from the concept of the presentinvention.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is: 1. A data processing system, comprising: anelectromagnetic transducer including a substantially thin, ferromagneticcore, said core having inner and outer edges defining concentric loops,and the major portion of said core defining a plane having a normal axiswithin said inner edge, gap defining means coupled to said core,intersecting both said loop defining edges and having juxtaposed polepieces shaped so as to provide two symmetrically disposed pairs ofopposed pole piece edges coextensive with said core plane and anelectromagnetic coupling means coupled to said core for convertingelectrical and magnetic signals; first transport means for transportinga magnetic data storage medium across one pair of pole piece edges;

second transport means for simultaneously transporting a magnetic datastorage medium across said second pair of pole piece edges, whereby saidmedia are simultaneously in operative engagement with saidelectromagnetic transducer.

2. The combination of claim 1 wherein said core consists of a singlelamina.

References Cited by the Examiner UNITED STATES PATENTS IRVING L. SRAGOW,Primary Examiner.

1. A DATA PROCESSING SYSTEM, COMPRISING: AN ELECTROMAGNETIC TRANSDUCERINCLUDING A SUBSTANTIALLY THIN, FERROMAGNETIC CORE, SAID CORE HAVINGINNER AND OTHER EDGES DEFINING CONCENTRIC LOOPS, AND THE MAJOR PORTIONOF SAID CORE DEFINING A PLANE HAVING A NORMAL AXIS WITHIN SAID INNEREDGE, GAP DEFINING MEANS COUPLED TO SAID CORE, INTERSECTING BOTH SAIDLOOP DEFINING EDGES AND HAVING JUXTAPOSED POLE PIECES SHAPED SO AS TOPROVIDE TWO SYMMETRICALLY DISPOSED PAIRS OF OPPOSED POLE PIECES EDGESCOEXTENSIVE WITH SAID CORE PLANE AND AN ELECTROMAGNETIC COUPLING MEANSCOUPLED TO SAID CORE FOR CONVERTING ELECTRICAL AND MAGNETIC SIGNALS;FIRST TRANSPORT MEANS FOR TRANSPORTING A MAGNETIC DATA